The policy shifts have been motivated by concerns over climate change and pollution caused by dirty engines.

London exceeded the European Union's annual limit for nitrogen dioxide exposure just five days into the new year in 2017. King's College, meanwhile, estimates that air pollution is responsible for 9,400 premature deaths in the city each year.

Mayor Sadiq Khan has made cleaner air one of his top priorities.

He has announced new "ultra low emission zones" that will be in place by September 2020. All vehicles entering the inner London will have to meet strict emission standards, or pay a special fee.